This invention relates to a process and composition for producing a permselective composite membrane. The permselective composite membrane is composed of a supported ultrathin film of a certain polybenzimidazolone which has superior water flux and salt rejection properties and finds a particularly advantageous use as a reverse osmosis membrane.
The permselective membrane is a membrane which has selective permeability to specified molecules. It is frequently used to remove very small amounts of contaminated molecules dissolved or diffused in a liquid or gas.
In recent years, reverse osmosis has attracted a great deal of interest for utilization in fields involving purification of liquids. This is of especial importance when utilizing this system in the purification of water and brackish water. Likewise, the process is also used to remove impurities from liquids such as water or, in the fields of dialysis, blood. When utilizing reverse osmosis in the purification of a brackish water, a pressure in excess of the osmotic pressure of the brackish water feed solution is applied to the solution which is prepared from purified water by a semipermeable membrane. Pure water thereby diffuses through the membrane while the sodium chloride molecules or other impurities which may be present in the water are retained by the membrane.
The efficiency of the reverse osmosis method is greatly affected by the properties of the permselective membrane used. Much effort has therefore been made to develop membranes having high performance, and resulted in some specific suggestions.
For example, U.S. Pat. Nos. 3,133,132 and 3,133,137 disclose the early Loeb-type membranes made of cellulose diacetate. These membranes are asymmetric membranes which are characterized by a very thin, dense surface layer or skin that is supported upon an integrally attached, much thicker supporting layer. These known membranes based on cellulose diacetate have the defect of poor compaction, low resistance to chemical and biological degradation, a short useful life, and insufficient flux and salt rejection characteristics.
U.S. Pat. No. 3,580,841 suggests an ultrathin semipermeable reverse osmosis membrane consisting of a cellulose acetate having a thickness between 0.05 and 0.50 micron.
Further, U.S. Pat. No. 3,648,845 suggests a process for making a thin semipermeable membrane composed of cellulose acetate which comprises depositing a film-forming solution of cellulose acetate in an organic solvent on a substantially non-porous buffer layer of polyacrylic acid coated on a microporous substrate, evaporating off the solvent, and at least partially removing the buffer layer by leaching. The membranes suggested in these U.S. Patents are also based on cellulose acetate, and cannot be free from the defect of poor compaction, low resistance to chemical and biological degradation, short useful life and insufficient flux and salt rejection characteristics as the membranes described hereinabove. The method of the last-cited U.S. Patent further requires a complicate procedure of pre-coating a buffer layer on a microporous substrate and then leaching the coating.
U.S. Pat. No. 3,587,832 suggests a permselective membrane based on a nitrogen-containing polymer which includes, for example, wholly aromatic polyamides and polyhydrazide. Typical examples of the nitrogen-containing polymer disclosed in this U.S. Patent are (meta-phenylene isophthalamide-terephthalamide) copolymer, and poly(isophthalic hydrazide). Permselective membranes obtained from these polymers are neither satisfactory in regard to salt rejection characteristics and chemical resistance.
In an attempt to remove these defects of the Loeb-type membranes, some membranes composed basically of synthetic polymers have recently been suggested. For example, U.S. Pat. No. 3,951,815 discloses a composite semipermeable membrane comprising a microporous substrate and an ultrathin film formed of a crosslinked, grafted polyethylenimine disposed on one surface of the microporous substrate that has been crosslinked with a di-or or tri-functional compound such as isophthaloyl chloride and grafted with a graft reactant such as acrylonitrile or epichlorohydrin. U.S. Pat. No. 4,005,012 describes a composite semipermeable membrane comprising an ultrathin film formed by contacting an amine-modified polyepihalohydrin with a polyfunctional agent on a microporous substrate to form this film on one surface of the microporous substrate. Also, U.S. Pat. No. 4,039,440 discloses a reverse osmosis membrane prepared in situ on a porous support by initial formation of a layer of polyethylenimine on the support, followed by interfacial reaction with a polyfunctional reagent to produce a thin surface coating which possesses salt barrier characteristics.
The membrane composed basically of crosslinked polyethylenimine disclosed in U.S. Pat. No. 4,039,440 has a high salt rejection, but has the defect of insufficient water flux and low oxidation resistance (e.g., low resistance to deterioration by the presence of chlorine in the feed saline or brackish water). As one method of improving the oxidation resistance, U.S. Pat. No. 3,951,815 suggests the grafting of acrylonitrile to the polyethylenimine. The acrylonitrile-grafted and cross-linked polyethylenimine shows some improvement in oxidation resistance, but suffers from the serious defect of markedly reduced water flux.
The membrane composed basically of the amine-modified polyepihalohydrin disclosed in U.S. Pat. No. 4,005,012 exhibits a high salt rejection but its water flux is not sufficient. It has been strongly desired therefore, to develop membranes having a higher water flux.
Basically the characteristics required of permselective membranes are high levels of permselectivity and flux. In addition, they should have high resistance to compaction, superior resistance to chemical and biological degradation, and sufficient flexibility to endure shaping into modules in actual use such as a tube, spiral or hollow filament. The membranes so far suggested lack one or more of these characteristics, and are not entirely satisfactory for use as permselective membranes.
Accordingly, the art has strongly desired to develop membranes having a combination of the aforesaid desired characteristics.
The present inventors previously discovered a nitrogen-containing aromatic polymer containing in the main chain of the polymer a nitrogen-containing cyclic group of the following formula ##STR1## wherein Ar represents an aromatic group, the two nitrogen atoms bonded to Ar being bonded to the ring carbon atoms at the ortho-position of the aromatic group; B represents a hydrogen atom or a bond to X in which case X is doubly bonded to N; and X represents an atomic grouping which forms a 5- or 6-membered ring together with the two nitrogen atoms and the two carbon atoms at the ortho-position of Ar to which the first two nitrogen atoms are handled, such as ##STR2## and disclosed that this polymer and a thin film prepared from it have superior permselectivity, and the nitrogen-containing aromatic polymer would be acceptable as a permselective membrane (see German Offenlegungsschrift No. 2524332) which corresponds to U.S. Pat. No. 4,085,090 to Hara et al.
The present inventors furthered their investigation into possible uses of the aforesaid nitrogen-containing aromatic polymers as permselective membranes, and consequently found that a membrane prepared from a specified imidazolone ring-containing aromatic polymer within the definition of the above-mentioned nitrogen-containing aromatic polymers has excellent permselectivity and high flux properties and also possess high resistance to compaction, superior resistance to chemical and biological degradation and sufficient flexibility to endure shaping into modules in actual use such as a tube, spiral or hollow filament, and is very suitable as a permselective membrane. The imidazolone ring-containing aromatic polymer is the one comprising at least 30 mole %, based on the entire recurring units, of at least one recurring unit of the following formula ##STR3## wherein A.sub.1 is an aromatic group having a valence of (3+l) the dashed line represents the presence or absence of a bond, and the two nitrogen atoms and W.sub.1 are all bonded to the ring carbon atoms of the aromatic group A.sub.1 ; A.sub.2 is an aromatic group having a valence of 2+ m); Y is --0--or ##STR4## in which R.sub.1 is a hydrogen atom or a monovalent hydrocarbon residue; and the two nitrogen atoms bonded to A.sub.1 are bonded to the nuclear carbon atoms at the ortho-position of the aromatic group A.sub.1 ; l and m are identical or different and each represent an integer of 0 to 3; when Y is ##STR5## and one of W.sub.1 groups is bonded to the nuclear carbon atom at the ortho-position of the aromatic group A.sub.1 together with Y, the W.sub.1, Y and A.sub.1 can form a 5-membered ring same as ##STR6## in which all symbols are the same as defined above; and when W.sub.1 does not form the 5-membered ring, W.sub.1 and W.sub.2 are identical or different and represent at least one group selected from mono-substituted nitrogen-containing groups derived from a primary amino group.
As is described in the above-cited prior art references, a permselective membrane for a reverse osmosis method is usually used in the form of a self-supporting anisotropic membrane, a laminate membrane obtained by laminating a self-supporting isotropic thin film to a microporous substrate, or a composite membrane consisting of a permselective ultrathin polymeric film supported on a microporous substrate. In the reverse osmosis method, a considerably high pressure of several kg/cm.sup.2 to several tens of kg/cm.sup.2 is usually exerted on the membrane. Accordingly, the anisotropic membrane or laminate membrane breaks during the operation or is compacted, leading to a marked reduction in flux. In order to remove this defect, attempts are frequently made to coat a microporous substrate having relatively high permeability to water with a solution of a polymer that gives permselectivity thereby to deposit an ultrathin film and provide a permselective composite membrane.
The present inventors, too, attempted to produce a permselective composite membrane by using a solution of the imidazolone ring-containing aromatic polymer. However, the amide solvents, such as N-methyl pyrrolidone, N-methyl caprolactam, N,N-dimethyl acetamide, hexamethyl phosphoramide or tetramethylurea, sulfoxide solvents such as dimethyl sulfoxide, and phenolic solvents such as phenol, cresol or chlorophenol, which are disclosed in the above-cited German OLS No. 2524332 as solvents capable of dissolving this aromatic polymer, dissolve most of the microporous substrates heretofore used as supports for permselective composite membranes and as ultrafiltration materials. Hence, it is impossible to coat a solution of the polymer in such a solvent directly on the substrate, and these solvents have been found to be useless in the production of permselective composite membranes.
Furthermore, the present inventors attempted to form an ultrathin film by coating a solution of the imidazolone ring-containing aromatic polymer in the aforesaid solvent on a microporous substrate which has been previously coated with a substantially nonporous buffer layer of polyacrylic acid in accordance with the method suggested in U.S. Pat. No. 3,648,845 cited above. However, the buffer layer was attacked by the solvent, and they failed to produce a satisfactory permselective composite membrane.
With a view to providing a permselective composite membrane having a supported ultrathin film of the imidazolone ring-containing aromatic polymer by coating a solution of the polymer on a microporous substrate, the present inventors extensively worked on solvents which would sufficiently dissolve the imidazolone ring-containing aromatic polymer without substantially dissolving the microporous substrate. Surprisingly, these efforts have led to the discovery that an aqueous medium containing a water-miscible amine having a pKa of more than 5.0 in a concentration of at least 5% by weight readily dissolves the imidazolone ring-containing aromatic polymer, but does not substantially dissolve the microporous substrate.
The term pKa, as used in the present specification and the appended claims, denotes a constant which characterizes the dissociation of a base to its conjugate acid in water at 25.degree. C. A detailed description of pKa is found, for example, in D. D. Perrin, "Dissociation Constants of Organic Bases in Aqueous Solution", Butterworths, London (1965).